Selective Chemical Stripping for Thin Metallic Finish Isolation
Selective finish stripping isolates thin gold or tin layers using ligand-accelerated redox baths while protecting nickel barriers through passivating inhibitors.
Capillary Undercut
Solder Morphology
A capillary undercut exists when molten metal pulls away from the vertical sidewall of a printed circuit board through hole during the cooling phase, creating a concave meniscus that leaves the copper barrel exposed. This phenomenon occurs during wave soldering or selective soldering processes when the thermal mass of the component lead prevents the wetting of the upper pad area. Operators detect the condition by cross-sectional analysis or high-magnification optical inspection.
The geometry governs the structural integrity of the joint by reducing the volume of filler metal in the critical fillet region. Its presence suggests an imbalance in the heat transfer profile across the laminate and the metallic interconnection. Boundary limits for the defect depend on the class of the assembly under inspection and the specific requirements for hole fill percentage defined by current IPC standards.
When the recession reaches the midpoint of the barrel, the joint fails to meet established workmanship criteria for signal reliability and mechanical attachment strength.
Thermal Geometry
The mechanism driving the formation of a capillary undercut begins with the rapid solidification of the solder alloy at the component side of the board. Heat flows away from the joint faster than the molten volume can replenish the gap through natural attraction forces. Fluid dynamics dictate that the surface tension of the cooling alloy pulls the liquid inward toward the mass of the lead and the primary land.
Low heat input results in a lack of flow toward the secondary side, forcing the metal to abandon the upper portion of the internal hole walls. Excessive flux activity or contamination further inhibits the metal from maintaining contact with the metallic surface as the cooling curve proceeds.
Defect Assessment
Validation of the joint quality relies on identifying the concave shape within the barrel rather than counting the total volume of metal. Engineers verify the presence of the defect by inspecting the transition zone where the solder meets the via wall. If the curvature remains within the specified threshold for the product category, the connection provides sufficient electrical conductivity for the intended operating environment.
Persistent occurrence of the condition requires adjustments to the preheat stage or the temperature of the molten reservoir. This failure mode represents a compromise in the metallurgical continuity of the hole filling process.